Lately, rubbers used for tires must have reduced hysteresis loss values in order to reduce energy consumption. Thus, natural rubbers (NR), polyisoprene rubbers (IR) and polybutadiene rubbers (BR) are used because they have low hysteresis loss values.
Some synthetic rubbers with lower hysteresis loss values are prepared by coupling after being formed by polymerization using organolithium compounds, the coupling being carried out with halogenated tin compounds (JP-A-57-55912). These polymers have good properties, in particular, reduced energy consumption.
In this method, a tin compound as a coupling agent, usually, is added after polymerization is completed, since this coupling process deactivates the active end of the polymer. If the polymer end is deactivated, unreacted monomers remain without reacting at all so that the molecular structure and the molecular weight of the resulting polymer is fixed at the time when the tin compound is added. So from an economical and molecular engineering point of view, the tin compound must be added at the end of the polymerization.
Another method for lowering the hysteresis loss value includes the use of polymers having a tertiary amine group at the end of the chains (JP-A-50-79590, JP-A-52-22484, etc.). In this process, initiators, for example, amide compounds of alkali metals, such as lithium dipropylamide, or cyclic lithium imide compounds, such as lithium piperidide, are prepared separately before use. In these processes, hysteresis loss values are lowered by introducing tin atoms into the polymers through coupling.
However, the processes described above require a coupling step after polymerization, which prevents continuous polymerization, therefore, reducing productivity and increasing cost. Also, the continuous processes known to those skilled in the art do not provide a polymer having both sufficiently improved properties and good processability. Moreover, these processes can not achieve 100 percent of coupling efficiency so that the resulting polymers do not have sufficiently good properties. Furthermore, the resulting coupled polymers have very high molecular weights which reduce processability.